Effect of Spherical Aberration onMSE Filters

December 5, 2005

Steve Scott & Jinseok Ko

MIT PSFC

Cambridge, MA

Thanks to Fred Levinton, Nova PhotonicsFile: spherical aberration.ppt

Effect of Spherical Aberration onMSE Filters

or

“Oh, never mind”

December 5, 2005

Steve Scott & Jinseok Ko

MIT PSFC

Cambridge, MA

Thanks to Fred Levinton, Nova Photonics

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Optical Assembly at MSE Filters

LensLens Filter

Detector Fiber optic4.5 x 6 mm

70 mm

= o ( 1 – (N1/N2)2 sin2 ) 0.5

= 0.15 nm for o = 658, = 2.45o., N1 = 1., N2 = 2.

Oven

spherical aberration causesexcessive focussing at the

edge of the lens

Thought Process (or: why I got excited)

• The narrowband filters used by MSE shift to the blue if the incident light is not normal. 5 degrees 0.63 nanometer shift.

• We use a lens to collimate the light from the fiber bundle onto the filter, to direct nominally collimated light onto the filter.

• Because the lens height is comparable to the focal length, it will suffer significant spherical aberration, particularly for rays that hit near the periphery of the lens.

• Ray-tracing for a fiber bundle at the focal point of the lens, including the its real size (4mm x 6 mm), indicates a mean shift of 1.2 nm and some rays experience a 10 nm shift, i.e. enormous.

There is a clever solution to this problem

• Shift the fiber bundle closer to the lens.

• From geometric optics, the emerging rays will diverge (virtual image)

• The amount of divergence can be chosen to approximately cancel the ‘overfocussing’ effect of spherical aberration.

• Can realize reductions of factor of 100 in angular spherical aberration using this approach – if light source is a point source on the optical axis.

• Even when a distributed light source (4mm x 6 mm) is considered, can still realize factor ~10 reductions in aberration.

But Fred Levinton already thought of it

• In the design of the PMT/filter housing implemented by Fred, the fiber bundle was moved forward by ~16 mm relative to the focal point.

• This is exactly the optimum shift suggested by my ray tracing.

• When the ray tracing calculations include the 4mm x 6 mm size of the fiber bundle, they suggest that we might see shifts of 0.2 – 0.5 nm, i.e. not necessarily negligible.

• We need to know the ‘illuminated area’ of the filter, which is determined by the angular spread of light hitting the fiber bundle at the dissector.

• Previous measurements of the filter function by Bill Rowan found blue shifts of 0.05 – 0.2 nm compared to the filter specs.

Conclusions

• We should close out this analysis by

• Ray-tracing the MSE optics to compute the angular distribution of rays striking the fiber bundle.

• Measure the size of the image spot on the filter or collimating lens.

• But it looks unlikely that spherical aberration is causing a serious problem for MSE.